Ultrasound imaging is an increasingly valuable medical tool for both diagnosis and therapy of human disease. For example, transrectal ultrasound imaging is employed to image the prostate in the treatment of prostatic cancer. In this application, a specially designed ultrasound imaging probe is placed in the rectum transanally after the rectum has been evacuated of stool and gas. The tip of the probe contains one or more ultrasound transducers transmitting and receiving at selected ultrasonic frequencies to image body tissues.
Regardless of the specific application, the optimal ultrasound images are obtained when there is a suitable liquid or semi-liquid interface between the tissue being imaged and the ultrasound transducer. This interface provides clear transmission paths for ultrasound waves emanating from the ultrasound transducer to the tissue and waves reflecting back from the tissue to the ultrasound transducer. The interface also helps to position the ultrasound transducer at distances from the tissue being imaged which fall within the range of focal lengths of the ultrasound transducer.
Most ultrasound probe manufacturers have developed a watertight sheath or condom with which to cover the ultrasound transducer at the tip of the probe. As illustrated in FIG. 1, a conventional sheath 203 is filled with fluid to provide the required liquid interface. Typically, this sheath encasing at the end of an ultrasound probe 201 forms a circumferential, essentially spherical, and omni-directional liquid tight balloon cover which is filled to the desired volume with water or saline. The liquid volume added will, to some extent, determine the separation of the ultrasound transducer in the probe from the tissue to be imaged and displace extraneous gas or stool fragments in the rectum which may interfere with the ultrasonic waves emanating from the ultrasound transducer.
All existing systems employing the conventional interface have a major deficiency. That is, the thickness and position of the interface in relation to the ultrasound transducer becomes inconsistent in an unpredictable manner, because the balloon of liquid surrounding the tip of the probe is deflected in an uncontrolled and undesirable way by gravity and contours formed by surrounding tissues. Movement of the probe tip within this fluid sack is thus largely variable, difficult to control, and may result in inferior images. The effects of gravity and the surrounding tissues are particularly troublesome when performing procedures in the lithotomy position usually used in transrectal ultrasound imaging for prostate biopsies under anesthesia, brachytherapy, or laser ablation.
U.S. Pat. No. 5,265,612 discloses a sheath for an intracavity ultrasonic device having regions of different elasticity. An area of the sheath near the ultrasound transducer is of greater elasticity, which facilitates localized deformation of a selected region of the tissue. As shown in FIG. 1 of this patent, other areas of the sheath still can expand under the fluid pressure. Moreover, manufacture of the sheath is difficult and, is not cost effective, given that it is desirable to have single-use, disposable sheaths. U.S. Pat. No. 5,623,940 discloses a catheter with a balloon which is inflatable out from an inflation window in a covering which encases all of the catheter. The diameter of the entire catheter assembly is substantially increased. As a result, insertion and removal of the catheter becomes more difficult and painful if the patient is awake.
Thus, there exists a need for an improved device for controlling the interface between an ultrasound imaging probe and the tissue being imaged.